Powder metallurgy is known to be performed as follows. That is, a material powder mainly made from a metal is compressed to form a compact, followed by heating such compact so as to sinter the same, thus obtaining a sintered body having a given shape. As for such sintered body which is obtained by compacting a material powder at first and then sintering the same, a high degree of freedom in product shape is ensured in a way such that products having relatively complex shapes can be manufactured at low cost (e.g. Patent document 1).
Particularly, as for parts such as various gears that are subjected to stresses when coming into contact with chains and other gears at the point of use, surface densification has been performed by reducing voids. In this way, the intensity of abrasion at points of contact can be lowered, and the strengths at such points of contact can be improved.
Meanwhile, in order to improve the precision in size that has worsened due to deformation at the time of performing sintering, there has been performed sizing in which recompression is performed by a press after performing sintering.
Examples of conventional methods for performing densification are as follows. That is, a high pressure may be applied when performing sizing; a sintered body that has softened through preliminary sintering may be recompressed; and there have also been attempted rolling, shot peening, cold forging and hot forging.
However, there has been a problem that a die will break easily if an excessively high pressure is applied at the time of performing sizing. Also, recompression after preliminary sintering, rolling, shot peening, forging and the like require more steps, which has led to a problem that cost will increase.
Here, as a surface densification method that is performed at the time of performing sizing and does not employ a high pressure, attempts that have been made include installing stepwise portions, protrusions and tapered portions in a sizing die, leaving a large ironing margin and performing coining (Patent documents 2, 3, 4 and 5).